The Americans are especially annoyed at the failure to establish clearly that the EU's military planning should be carried out by Nato's Supreme Headquarters Allied Powers Europe (Shape). Mr Cohen added: "There should be no separate "EU caucus within Nato". His outburst, contained in his valedictory speech to Nato before leaving office next month, has exposed Tony Blair to renewed Tory criticism that his European defence initiative will weaken or even split Nato.

The row will cast a pall over the EU summit, which opens in Nice tomorrow, when the Europeans are supposed to unveil their proposals for permanent institutional defence links between the EU and Nato. Departing from his speech, delivered behind closed doors, witnesses said Mr Cohen asked a series of pointed questions.

Were the Europeans serious about raising defence spending and improving their ability to project military power?

Would they clearly spell out the role of Nato's Deputy Supreme Allied Commander Europe as the effective operational head of the EU force?

Will any new military capability be shared with Nato? And what if the agreed policy is undermined by "trickiness"?

Mr Cohen said: "If all these factors are not taken into account, then Nato could become a relic of the past." Britain was quick to say that it shared Mr Cohen's concerns. Geoff Hoon, the Defence Secretary, said: "It is very strongly the view of the British Government that planning should be done within Nato because that is where the expertise is. Otherwise it would risk duplicating facilities and that would cause the EU and Nato to pull in different directions."

Lord Robertson, the Nato Secretary General, added that a separate planning staff would waste resources. The Government had hoped to end the debate on its European defence policy with an EU statement, to be issued at Nice. This is supposed to spell out that Nato remains the basis of collective defence and does not involve the establishment of a European army.

The document, said Robin Cook, the Foreign Secretary, "deflates the myths and replaces them with the facts". However, US officials said the document still gives room for the EU to set up a separate strategic planning staff outside Nato. The US has supported the idea of a 60,000 strong rapid reaction force in the hope that it would increase Europe's share of the burden within Nato under the guise of European integration.

On the flight to Brussels, Mr Cohen told reporters that a separate operational planning capability would "weaken the ties between the United States and Nato and Nato and the EU". Mr Cohen later toned down his comments. "The Americans are not worried about a European army, they are worried about a phantom army. It's all about capability."

The main unspoken target of his attack is France, which is still trying to maintain a separate planning capacity. In an addition to the Nice treaty, Paris wants to bring defence under the area of the EU's "enhanced cooperation" that can go ahead with eight rather than all 15 members.

The public is opposed to British troops becoming part of a new defence force, according to a poll commissioned by the Conservatives.

The ICM poll asked more than 1,000 people if they thought that "substantial numbers of British troops should be put into a European military force under the command of the EU". Fifty-six per cent said no, 30 per cent agreed they should.